Craft the Vinaigrette : In a small jar with a lid, combine the extra-virgin olive oil, lemon juice, Dijon mustard, and dried oregano. Secure the lid and shake vigorously until the dressing is well-emulsified. Season with a pinch of salt and pepper.
Prepare the Salad Base : In a large mixing bowl, gently toss the mixed greens, grape tomatoes, English cucumber, artichoke hearts, Castelvetrano olives, and sliced red onion. Set aside.
Season the Salmon : Thoroughly pat the salmon fillet dry with a paper towel. This ensures a perfect sear. Sprinkle the flesh side evenly with the garlic powder, ¼ teaspoon of the sea salt, and ⅛ teaspoon of the black pepper.
Pan-Sear the Fillet : Place a cast-iron pan over medium-high heat. Add the avocado oil. Once the oil shimmers, carefully place the salmon in the pan, flesh-side first. Cook for 6 minutes, resisting the urge to move it.
Flip and Finish : Using a fish spatula, flip the salmon. The skin should release easily. Cook for another 3 to 4 minutes, or until the salmon's internal temperature reaches 135°F (57°C) for a medium, flaky finish.
Rest and Flake : Transfer the cooked salmon to a clean cutting board and let it rest for 5 minutes. Use two forks to gently break the fillet apart into large, rustic flakes, discarding the skin if desired.
Assemble and Serve : Drizzle about half of the prepared vinaigrette over the salad base and toss to coat lightly. Arrange the greens on a platter, top with the flaked salmon, and sprinkle with feta cheese. Serve immediately, passing the remaining dressing at the table.